Generally I'm doing a lot of sports, (volleyball, hiking, soccer, swimming, etc.) and while hiking I have no problem to do 3000 meters or more in a day (uphill ;)) but when I go mountain-biking I have some problems. I can cycle the first 500-600 altitude difference more or less in a normal speed (~1 hour), but afterwards if I should cycle further I'm exhausted.

So whats the best way to improve my stamina? Should I practice every day a little bit? Or is it better to sometimes go for long tours?

Basically, Carmichael's methods are a form of interval training where you are doing things like hill-repeats, power intervals, fast pace intervals, etc. There's a lot of info out there on interval training. One of the more common is called HIIT (High Intensity Interval Training).

Another resource relating to interval training is here on Dr. Mirkin's web site where he discusses how sprinting improves endurance.

FWIW - I've made interval training my primary method of training for long distance riding. I do multiple centuries during the summers and rarely do training rides over 60 miles. Prior to 4 years ago, my mode of training for long distance was a gradual distance build-up and had mixed results. With interval training, my performance on long distance rides has improved significantly in both speed and how I feel afterwards. Seriously, this plan works.

The climbing that kills me when mtbing are the explosive short climbs, not the long grinds. If that is the case with you, I would suggest some sort of interval training. This sort of training should help your recovery speed.

Also, make sure that you are taking nutrients in while exercising. Low to moderate exertions can be fueled by body fat, but high intensity needs to tap into your carbo reserves, and I find that mine get depleted quickly without having some sort of replacement ready.

I don't know that it's good, nor even possible, to 'train' when you're 'exhausted'? So, why are you exhausted, and how to avoid it: perhaps you need to rehydrate or to refuel, perhaps on the fly; or perhaps your 'normal speed' for the first hour needs to be slower/easier (even for the fittest, the 'normal speed' for a sprint isn't the same as for a marathon).

FWIW I've been cycling for a bit over an hour twice a day, 3-5 days/week for the last month (after eating, so I don't run out of fuel on the way). And I haven't been pushing myself too hard (e.g. I'm using low gears when I get to a hill) because I don't want to strain my knees: and I'm sure I'm getting stronger/more enduring.

If strength is about increasing muscle, perhaps endurance is about increasing vascularisation. Apparently when you exercise your "VO2max" i.e. the rate at which you can burn/use/take in oxygen increases, and that's partly because you get more capillaries (I imagine, I don't know, that it's blood vessels that were previously too small to be effective that increase to meet the demand ... perhaps it's new vessels too); maybe that (growth and change in tissues) can't happen overnight.

So whats the best way to improve my stamina? Should I practice every day a little bit? Or is it better to sometimes go for long tours?

Apparently someone once asked a champion whether it's better, in a race, to push hard and slowly in a high gear, or to spin more easily and quickly in a lower gear: and he thought about that and said, "It's better to push hard and quickly in a high gear."

Similarly (and here again, I'm guessing) improving stamina might not be to do with practicing either often or for a longer time, but both.

Fartlek training is a form of interval training, of which there are a number of variations. Chris Carmichael's system of interval training is specifically designed for cycling; both racing and long distance although the racing and distance programs differ somewhat. The origin of interval training is generally attributed to Dr. Woldemer Gerschler of Germany in 1930.
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user313Jul 5 '11 at 6:47

Are you eating while you ride? Consuming carbs in a sports drink? If not, you're probably bonking due to lack of sugars after your glycogen reserves are depleted. If it seems like you just hit a wall at the same time each time you ride, be sure to Eat before your ride, and during to maintain fuel for your muscles.

I'll usually throw in a cliff bar or something every 8 or 10 miles of singletrack. Less frequently on a road bike. But after a few good steep climbs, I make sure I get some calories (even though I'm not hungry), to keep my energy level up.

Thanks for the answer. While I'm riding I'm eating from time to time, mainly bananas. Before the ride I can't eat anything, because then I feel sick. I don't eat before any sport (soccer, hiking, volleyball, jogging, swimming, etc.)
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RoflcoptrExceptionMay 3 '12 at 17:13

Maybe you want to supplement with a few calories in your drink in addition to the bananna or Possibly a few more bites to eat like a 1/2 pb&j or something similar.
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BenzoMay 3 '12 at 18:18